Electrolocation apparatus and methods for providing information about one or more subterranean feature

ABSTRACT

In some embodiments, a method of approximating or determining at least one dimension of at least one underground geological feature in a zone of interest proximate to a well bore includes generating an electric field in the zone of interest while the well bore and geological feature at least partially contain conductive fluid. At least two sensing electrodes are provided in the well bore and configured to detect differences therebetween in electric potential caused by at least one target object in the zone of interest and provide data relating thereto to at least one data processing system. The data processing system(s) approximates or determines the dimension(s) of the geological feature(s) based at least partially upon data provided by the sensing electrodes.

This application is a continuation-in-part of and claims priority toU.S. patent application Ser. No. 12/421,061, filed Apr. 9, 2009 andentitled “Electrolocation Apparatus and Methods for Mapping from aSubterranean Well”, which claims priority to U.S. provisional patentapplication Ser. No. 61/044,153, filed Apr. 11, 2008 and entitled“Electrolocation Technique for Hydraulic Fracture Mapping”, both ofwhich are hereby incorporated by reference herein in their entireties.

FIELD OF THE INVENTION

The present disclosure relates generally to assessing geometry and othercharacteristics in and around subterranean wells and includes, forexample, embodiments for estimating at least one dimension or othercharacteristic of an underground geological feature.

BACKGROUND OF THE INVENTION

In various operations conducted via underground wells, it is oftenadvantageous to be able to gain information about one or more variablesexisting in the well bore or subterranean formation. In the arena ofhydrocarbon exploration and production, example operations during whichit may be beneficial to gain such information are drilling, cementing,completion, stimulation (including well treatment) and workover. Thevariables could be any measurable condition, parameter or property, suchas one or more geometric dimension, the location of a particular objector geometric feature, temperature, pressure, flow, chemical composition,in-situ stresses in the well bore or formation, or the like. Note, thepresent disclosure is not limited by the type of operation, the targetlocation in the well bore or formation or the type of variable.

In one example in the hydrocarbon recovery arena, it is often ofsignificant benefit to learn about the geometry of an area within anunderground well or subterranean formation, such as the location ordimensions of hydraulic fractures. Hydraulic fracturing is a widely usedprocess for stimulating oil and gas wells and which typically involvesinjecting fluid into the well bore at a higher pressure than thesurrounding reservoir pressure. The higher pressure of the injectedfluids causes the formation to fracture, exposing the surface areathrough which oil or gas may flow.

Once hydraulic fractures are formed, it is believed to be advantageousto learn about or map out the geometry or dimensions of the fractures,such as to increase the effectiveness of the fracturing process andhydrocarbon production. For example, proppant is typically deliveredinto the fracture during well treatment to effectively increase theconductivity of the fracture and provide a flow path of hydrocarbonsbetween the reservoir and the producing well bore. Proppants ensure thecreated flow path remains open and conductive after the treatingpressure is relieved. Proper placement of the proppant is oftenconsidered one of the most critical facets of fracture stimulation. Thepropped fracture area is believed to directly correlate with stimulatedwell potential and productivity, whereby the larger the proppedfracture, the more productive the well. It is thus typicallyadvantageous to know the location and/or dimensions of propped fracturesor proppant packs within conductive fractures. For example, knowledge ofthe location of proppant in fractures and/or the dimensions of proppedfractures can, in some instances, assist in optimizing and improvingfracturing efforts and proppant distribution, well placement andproduction strategies.

Existing techniques for visualizing underground geometries, such ashydraulic fracture propagations, include micro-seismic fracture mapping,tilt-meter fracture mapping and the use of tracers. These techniques arebelieved to have one or more drawbacks or limitations. For example, someof these techniques are believed to be limited to representing only onedimension of fracture geometry (e.g., length (depth), height orazimuth). For other examples, some of the current mapping technologiesrequire the use of an offset well, which may dramatically increasecosts, and/or radioactive material, which may be environmentallydamaging.

It should be understood that the above-described discussion is providedfor illustrative purposes only and is not intended to limit the scope orsubject matter of this disclosure, the appended claims or the claims ofany related patent application or patent. Thus, none of the appendedclaims or claims of any related patent application or patent should belimited by the above discussion or required to address, include orexclude the above-cited examples, features and/or disadvantages merelybecause of their mention above.

Accordingly, there exists a need for improved systems, apparatus andmethods capable of estimating at least one dimension or othercharacteristic of an underground well or geological feature having oneor more of the attributes, capabilities or features described or claimedbelow or evident from the appended drawings.

BRIEF SUMMARY OF THE DISCLOSURE

In some embodiments, the present disclosure involves a method ofapproximating or determining at least one dimension of at least onegeological feature of an earthen formation. Each such geological featureis at least partially located within a zone of interest in the earthenformation proximate to a subterranean well bore. While the well bore andgeological feature at least partially contain conductive fluid, themethod includes generating an electric field in the zone of interest. Atleast one target object is located in the zone of interest, has anelectrical impedance that differs from that of the conductive fluid andcreates perturbations in the electric field. At least two sensingelectrodes are provided in the well bore. The sensing electrodes areconfigured to detect differences therebetween in electric potentialcaused by the target object(s). At multiple different times and multipledifferent locations in the well bore, the sensing electrodes detectdifferences therebetween in electric potential caused by the targetobject(s) and provide data relating thereto to at least one dataprocessing system. The data processing system approximates or determinesat least one dimension of the geological feature in the earthenformation based at least partially upon data provided by the sensingelectrodes, whereby the dimension(s) of at least one geological featureis approximated or determined with the use of electrolocationprinciples.

The present disclosure includes embodiments of a system forapproximating or determining at least one dimension of at least onegeological feature of an earthen formation from a subterranean wellbore. The geological feature is at least partially located within a zoneof interest in the earthen formation that is proximate to the well bore.The system includes a conductive fluid disposed in at least part of thewell bore and geological feature. At least two spaced-apart electricfield generating electrodes are configured to create a constant electricfield in at least part of the zone of interest. At least one targetobject is disposed within the zone of interest outside the well bore,has a different electrical contrast as compared to the conductive fluidand is capable of creating perturbations in the electric field. At leasttwo spaced-apart sensing electrodes are disposed within the well boreand configured to detect differences therebetween in electric potentialmeasured in volts caused by the target object(s) and provide datarelating thereto for use in approximating or determining at least onedimension of the geological feature(s). The sensing electrodes areconfigured to be movable relative to the target object(s) in order todetect electric potential differences at different times and locations.

Accordingly, the present disclosure includes features and advantageswhich are believed to enable it to advance underground investigation ormapping technology. Characteristics and potential advantages of thepresent disclosure described above and additional potential features andbenefits will be readily apparent to those skilled in the art uponconsideration of the following detailed description of variousembodiments and referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The following figures are part of the present specification, included todemonstrate certain aspects of various embodiments of this disclosureand referenced in the detailed description herein:

FIG. 1 is a schematic diagram showing an exemplary electrolocationsystem in accordance with an embodiment of the present disclosure;

FIG. 2A is a perspective view of a wing of an exemplary bi-wingsymmetrical hydraulic fracture formed in an earthen formation throughhydraulic fracturing;

FIG. 2B is a schematic diagram showing the length, height and width ofthe hydraulic fracture wing shown in FIG. 2A;

FIG. 3A is a schematic diagram of an unperturbed planar electric fieldshown to illustrate principals applicable in accordance with anembodiment of the present disclosure;

FIG. 3B is a schematic diagram of the planer electric field of FIG. 3Athat is perturbed by a circular target object centered at x=20 andy=−20;

FIG. 4 is a front view of an example testing tank with experimentalsandstone pieces used in a first example experiment to illustrateprincipals applicable in accordance with an embodiment of the presentdisclosure;

FIG. 5 is a two-dimensional iso-potential map from a three-dimensionalgraphical representation showing results generated in the first exampleexperiment;

FIG. 6 is a schematic diagram showing the placement of an exampleelectrode assembly relative to a hole formed in a sandstone piece usedin a second example experiment to illustrate principals applicable inaccordance with an embodiment of the present disclosure;

FIG. 7 is a two-dimensional iso-potential map from a three-dimensionalgraphical representation showing results generated in the second exampleexperiment;

FIG. 8 is a graph showing results and predicted bounds deduced therefromfrom the second example experiment; and

FIG. 9 is a graph of results from a third example experiment toillustrate principals applicable in accordance with an embodiment of thepresent disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Characteristics and advantages of the present disclosure and additionalfeatures and benefits will be readily apparent to those skilled in theart upon consideration of the following detailed description ofexemplary embodiments of the present disclosure and referring to theaccompanying figures. It should be understood that the descriptionherein and appended drawings, being of example embodiments, are notintended to limit the claims of this patent application, any patentgranted hereon or any patent or patent application claiming priorityhereto. On the contrary, the intention is to cover all modifications,equivalents and alternatives falling within the spirit and scope of theclaims. Many changes may be made to the particular embodiments anddetails disclosed herein without departing from such spirit and scope.

In showing and describing preferred embodiments, common or similarelements are referenced in the appended figures with like or identicalreference numerals or are apparent from the figures and/or thedescription herein. The figures are not necessarily to scale and certainfeatures and certain views of the figures may be shown exaggerated inscale or in schematic in the interest of clarity and conciseness.

As used herein and throughout various portions (and headings) of thispatent application, the terms “invention”, “present invention” andvariations thereof are not intended to mean every possible embodimentencompassed by this disclosure or any particular claim(s). Thus, thesubject matter of each such reference should not be considered asnecessary for, or part of, every embodiment hereof or of any particularclaim(s) merely because of such reference. The terms “coupled”,“connected”, “engaged” and the like, and variations thereof, as usedherein and in the appended claims are intended to mean either anindirect or direct connection or engagement. Thus, if a first devicecouples to a second device, that connection may be through a directconnection, or through an indirect connection via other devices andconnections.

Certain terms are used herein and in the appended claims to refer toparticular components. As one skilled in the art will appreciate,different persons may refer to a component by different names. Thisdocument does not intend to distinguish between components that differin name but not function. Also, the terms “including” and “comprising”are used herein and in the appended claims in an open-ended fashion, andthus should be interpreted to mean “including, but not limited to . . ..” Further, reference herein and in the appended claims to componentsand aspects in a singular tense does not necessarily limit the presentdisclosure or appended claims to only one such component or aspect, butshould be interpreted generally to mean one or more, as may be suitableand desirable in each particular instance.

Referring initially to FIG. 1, an electrolocation system 5 in accordancewith an embodiment of the present disclosure includes at least twoelectric field generating electrodes 10 and at least one sensingelectrode 15 disposed within, or proximate to, an underground well bore25. In this example, the well bore 25 is part of an open-hole(non-metallic cased) well or well-section. Fluid 28 is provided in thewell bore 25, which is surrounded generally by an earthen formation 30.At least one target object 35 is at least partially located within azone of interest 45 of the formation 30 adjacent to or near the wellbore 25.

FIG. 1 illustrates exemplary electrodes 10 configured to create anelectric field that reaches the zone of interest 45 and shows exemplarycontour lines of equal electric potential 18. The sensing electrode(s)15 detect perturbations 20 in the electric field caused by the targetobject(s) 35. To create the electric field, one or more of the electricfield generating electrodes 10 is positively charged and thus serves apositive electric field generating electrode (e.g. electrode 12) andanother one or more of the electric field generating electrodes 10 isnegatively charged and thus serves as a negative, or return, electricfield generating electrode (e.g. electrode 14). However, in someembodiments, any of the electrodes 10 may serve as either a positiveelectric field generating electrode 12 or a negative electric fieldgenerating electrode 14 and may change or alternate polarity duringusage. Based upon data collected by the sensing electrodes 15 and withthe application of electrolocation principals, at least one dimension orother characteristic of at least one portion of, or feature in, theformation 30 in the zone of interest 45, the well bore 25 or a geometricinterface therebetween may be determined or estimated.

Any suitable conductive fluid 28 is provided in the well bore 25 toallow the electric field to be created and perturbations to bedetectable. For example, the fluid 28 may be fracturing fluid introducedduring hydraulic fracture formation.

Without being limited by theory, “electrolocation” is known to generallyinvolve an electric field generated in a medium and objects located inthe medium. The objects differ in impedance from the medium and otherobjects therein, and create distortions, or perturbations, in theelectric field that can be detected by sensors. It should be noted,however, that while this general concept or process is referred toherein as “electrolocation”, it may be referred to with the use of anyother suitable terms, such as “electrosensing” and the like. Thus, theuse of the term “electrolocation” is not intended to and should not beconstrued to be limiting upon the present disclosed or appended claims.The detected data can be used to estimate or determine one or morecharacteristic of the objects such as, for example, the location of theobjects. Further details about electrolocation and exampleelectrolocation techniques, systems, applications and mathematicalmodels relating thereto may be found in various publicly accessibledocuments and sources, including, without limitation, the article Emdeet al., “Electric Fish Measure Distance in the Dark,” Nature, vol. 395,pgs. 890-894 (Oct. 29, 1998), the article Solberg et al., “RoboticElectrolocation: Active Underwater Target Localization with ElectricFields,” Proceedings of the 2007 International Conference on Roboticsand Automation (ICRA), Apr. 10-14, 2007, Rome, Italy, pp. 1-16, and thearticle Solberg et al., “Active Electrolocation for Underwater TargetLocalization,” The International Journal of Robotics Research, Vol. 27,No. 5, May 2008, pp. 529-548, each of which is hereby incorporated byreference herein in its entirety. However, the present disclosure is notlimited to the details, techniques or applications disclosed in any ofthe above or any other references.

The electrolocation system 5 of the embodiment of FIG. 1 is locatedin-situ in the well bore 25 without requiring the use of an offset well(not shown). The electrodes 10, 15 of the illustrated system 5 aredisposed on a carrier 38, such as a wireline 40, which can be insertedinto, moved within and removed from the well bore 25. However, theelectrodes 10, 15 may be positioned or delivered in any other suitablemanner. For example, some or all of the electrodes 10, 15 may bedisposed upon one or more pads (not shown) that are movable within thewell bore 25. For another example, some or all of the electrodes 10, 15may be carried on one or more drill string or other pipe, coiled tubing,tool or other component (not shown) moveable within the well bore 25. Inyet other embodiments, some or all of the electrodes 10, 15 may beattached to or embedded in a casing (not shown) disposed in the wellbore 25. For example, the electrodes 10, 15 may be affixed to the outerdiameter of the casing. When some or all of the electrodes 10, 15 areembedded in or affixed to metallic casing, special arrangements ofelectrodes 10, 15 may be required to be able to measure perturbationscaused by the target object(s) 35. Evaluating through-casing resistivityfor measuring current leakage may be necessary to sufficiently removethe influence or effect of the casing for accurate data recovery andanalysis. For yet another example, some or all of the electrodes 10, 15may be embedded in or affixed to a non-conductive casing, or casing thatdoes not strongly disturb the electric field, such as a compositecasing. In other embodiments, some or all of the electrodes 10, 15 maybe attached to or embedded in another component, fixture or areaproximate to the casing, such as insulation or cement.

Referring still to FIG. 1, the electric field generating electrode(s) 10may have any desired dimensions, form, construction, configuration,arrangement and operation suitable for creating an electric field.Without limitation, examples of suitable electric field generatingelectrodes 10 may include metal pieces or wire constructed of or coatedwith silver, gold, or other highly conductive materials. In theillustrated embodiment, the system 5 includes one positive electricfield generating electrode 12 and one negative electric field generatingelectrode 14. However, in other embodiments, the system 5 may includeany desired quantity of positive and negative electric field generatingelectrodes 12, 14. For example, the system 5 may include a singlepositive electric field generating electrode 12 (not shown), such as anelongated highly conductive metal piece, along with a multitude ofnegative electric field generating electrodes 14, all disposed withinthe well bore 25. For another example, one or more positive electricfield generating electrodes 12 may be disposed within the well bore 25and one or more negative electric field generating electrodes 14 may belocated at or near the earth's surface (not shown) or another locationoutside the well bore 25.

The electric field generating electrodes 10 may be spaced apart by anydesired distance to provide the desired electric field. Generally, thesize of the electric field and depth of penetration into the formation30 may be affected by the quantity and/or relative positioning of theelectric field generating electrodes 10. Moreover, the actualarrangement of electrodes 10, 15 may depend upon the particularapplication. For example, in some applications, a large number ofelectric field generating electrodes 10, such as ten, fifteen, twenty,thirty, etc., disposed along and spaced apart on a single wire, or otherelongated carrier, in the well bore 25 will provide a larger depth ofelectric field penetration than a pair of electrodes 10 disposed atopposite ends of the wire.

Still referring to FIG. 1, the sensing electrode(s) 15 may have anydesired dimensions, form, construction, configuration, arrangement andoperation suitable for detecting perturbations 20 in the createdelectric field. Without limitation, examples of suitable sensingelectrodes 15 may include metal pieces or wire constructed of or coatedwith silver, gold, or other highly conductive materials. In otherembodiments, the sensing electrodes 15 may be more sophisticated (evenmulti-purpose) sensors, as are and become further known. In thisembodiment, the system 5 includes two sensing electrodes 15. However, inother embodiments, the system 5 may include only one, or more than two,sensing electrodes 15. For example, in some embodiments, the system 5may include a multitude, such as twenty, sensing electrodes 15. (Itshould be noted that in embodiments involving only one sensing electrode15, it may be necessary or desirable to include at least one ground(e.g. metallic casing) or reference voltage disposed within or proximateto the well bore 25.)

The exemplary sensing electrodes 15 may be positioned at any desiredlocation suitable for detecting perturbations 20 in the created electricfield. In the illustrated embodiment, the sensing electrodes 15 aresuitably positioned in the well bore 25 in or along the zone of interest45 in the formation 30 to be able to detect perturbations 20 in theelectric field caused by the target object(s) 35. In another embodiment,for example, a system 5 may include multiple pairs of electric fieldgenerating electrodes 10 spaced apart in a single line, such as by 1″,and multiple sensing electrodes 15 offset relative thereto.

Still referring to the example of FIG. 1, the target objects 35 may belocated in the zone of interest 45 in any suitable manner and includeany desired structure, object or material capable of creating detectableperturbations in the electric field useful to approximate or determinethe desired dimension(s) or other characteristics of the subjectgeological feature or area. For example, the target object(s) 35 may bepart of the earthen formation 30, such as one or more hydraulicfractures or fracture surfaces (not shown), that will display asufficient electrical (resistivity) contrast to everything else in thewell bore 25 and/or zone of interest 45 (e.g. fluid, proppants, rock) toproduce meaningful perturbation data. For another example, the targetobject(s) 35 may be one or more object(s) or material(s) positioned inone or more hydraulic fracture(s) and which possesses an electricalimpedance that differs from the impedance of the fluid 28 and/or otherobjects and structures in the well bore 25 and zone of interest 45. Insome instances, the target objects 35 may be proppant placed into thehydraulic fracture(s) and which have sufficient electrical contrast. Asused herein, the term “proppant” includes any substance, composite orfluid-particle mixture useful for assisting in propping open a fracture,crack or other area in an underground earthen formation, or otherdesired purpose in a well bore or subterranean formation. While thepresence of proppant in the fracture may not be necessary to measureperturbations of various target objects, it may, in at least somesituations, make the perturbations stronger and thus assist in obtaininguseful data. When proppant is relied upon to conduct electric current,the proppant particles should preferably be in contact with one anotherand the fracture wall.

In yet other embodiments, the target objects 35 may be material orparticles contained within or carried by fluid 28 or proppant providedin the well bore 25. Some examples are beads constructed of or coatedwith metal, plastic or other material capable of raising or loweringelectrical conductivity, as may be desired depending upon the particularwell conditions. In even other embodiments, the target objects 35 mayinclude particles, such as, for example, nanoparticles, suspended in thefracturing or other fluid in the well bore 25.

In any case, any suitable material may be used as target objects 35 oraltered to provide a sufficient difference in electrical impedance ascompared to the fluid 28, formation 30 and other material and objects inthe well bore 25 and zone of interest 45 to create perturbations 20 inthe electric field that may be detected by the sensing electrodes 15.Likewise, if desired, the target objects 35 may include a combination ofthe above or other examples. Moreover, different target objects 35 maybe used at different times during operation of the system 5. Forexample, in some applications, perturbations or electric potential maybe measured from a well bore 25 before a fracture is formed (when theremay be no target objects present), during or after formation of thefracture and after fracture closure (with proppant in the fracture),each scenario potentially involving different target objects 35.

If desired, the target objects 35 may have a tunable, or variable,electrical impedance and thus be functionalized target objects.Increasing or decreasing the impedance of the functionalized targetobject could be useful to cause a desired interaction with the electricfield and improve data accuracy based upon particular conditions in thewell bore 25. Target objects 35 may be functionalized in any suitablemanner. For example, particles having a desired impedance may be addedto the target objects 35 to make them functionalized target objects. Insome embodiments, the particles may be coated onto, integrated into ormixed with the target objects 35. For one example, when the targetobjects 35 include nanoparticles suspended in fracturing fluid, ironnanoparticles may be added to increase the conductivity and decrease theresistance of the target objects 35 and provide the desired interactionwith the electric field.

The electrolocation system 5 of FIG. 1 is configured to provide data formapping or estimating one or more dimensions or other characteristics ofat least one area or geological feature proximate to the well bore 25and/or in the adjacent formation 30. In one exemplary application, thetarget objects 35 are located at one or more hydraulic fracture (notshown) formed in the formation 30 in the zone of interest 45.Perturbations 20 in the electric field caused by the target objects 35are detected and used to deduce at least part of the geometry of thefractures. For example, one or more among the approximate length(depth), width, height and azimuth of one or more fracture may be atleast partially deduced from the output of the sensing electrodes 15. Instill further embodiments, the deduced geometry may include the entiregeometry of the subject fracture(s).

For example, FIG. 2A illustrates one wing (referred to as “fracture” 32)of a bi-wing symmetrical hydraulic fracture formed in the earthenformation 30 through hydraulic fracturing. In accordance withembodiments of the present disclosure, at least one dimension of thefracture 32 may be determined or estimated using electrolocation. Forexample, referring to FIG. 2B, the approximate width W and/or height Hof the fracture 32 may be estimated.

For another example, in some embodiments, the length (depth) L of thefracture 32 may be estimated. (It should be noted that length L of FIG.2B represents the length of one wing of the actual exemplary bi-wingfracture.) To provide a sufficient electric field to estimate length Lof a fracture or other geological feature using electrolocation, aspecial arrangement of electrodes 10 and/or 15 may be necessary. Forexample, it may be necessary to expand the positioning and/or quantityof the positive electric field generating electrodes 12. In someapplications, the depth of penetration into the formation 30 of theelectric field by the positive electric field generating electrodes 12may be equal to the length of the full line of electrodes 12 or twicethat value. A multitude of positive electric field generating electrodes12, such as twenty, may be spaced-apart in a single long line anddirectionally oriented toward the fracture 32 to synchronously injectcurrent in parallel in the same direction to extend the full length L ofthe fracture 32. In other embodiments, a single elongated positiveelectric field generating electrode 12 may be used. A special electrodecarrier (not used) may be necessary, such as to directionally orazimuthally arrange the electrodes 12. For example, the carrier mayinclude one or more metal sheet shaped into a 30 or 60 degree angle. Inany case, the necessary quantity and arrangement of the electrodes 12may depend upon the particular application. Further, it may also benecessary to determine other variables, such as the volume, surfacearea, width W and/or height H of the fracture 32, in order to estimatelength L.

However, the present disclosure is not limited to determining theabove-described dimensions of hydraulic fractures. Other exemplaryfeatures that may be measured or mapped in accordance with the presentdisclosure are naturally occurring fractures, wormholes or channelscreated by matrix stimulation, and the like.

In an example operation of the embodiment of FIG. 1, any suitable fluid,such as fracturing fluid or a brine, is provided into at least part ofthe well bore 25 and zone of interest 45. The exemplary electric fieldgenerating electrodes 10 are spaced a pre-established or desireddistance apart and disposed at a suitable location in the well bore 25to provide an electric field in the zone of interest 45. The sensingelectrodes 15 are spaced apart a pre-established, or desired, distanceand disposed at a suitable location in the well bore 25 or formation 30to detect perturbations of desired target objects 35 (or the absencethereof) in the zone of interest 45. The perturbations may, for example,represent a detected change in electrical impedance caused by structuresof the formation 30, such as fractures formed therein, or materialplaced in the fractures, such as proppant or nanoparticles, such asdescribed above.

In another sample operation of the embodiment of FIG. 1, the carrier 38is lowered to a desired position (e.g. at or near the bottom or toe ofthe well bore 25) and a multitude of readings taken as the carrier 38 ismoved axially and/or rotationally in the well bore 25. Perturbationreadings are taken at different positions of the electrodes 10, 15relative to the target object(s). In some cases, this process isrepeated at different times in the development of the well or earthenformation, such as before fracturing the formation 30 (before or duringthe pumping of fracturing fluid into the well bore 25), when there maybe no target objects 35, and after fracture closure. If desired,perturbations may be measured several times while fracturing fluid (orother conductive fluid or material) is penetrating the fracture, such asto assist in tracking the propagation thereof or identify the locationand/or movement of the front end of the fracturing fluid (or otherconductive fluid or material) in the fracture. Electric potentialreadings may be taken by the electrodes 15 prior to creation ofgeological features (e.g. hydraulic fractures), or in portions of thezone of interest 45 or other sections of the earthen formation, wherethere are no geological features. In some cases, there may be no targetobjects 35 present in the measured region and the electric potentialdifference determination may be zero Volts.

In many embodiments, the more readings taken by the electrodes 15 andthe more positions of the electrodes 10, 15 for such readings, thegreater the accuracy, usefulness and resolution of the resulting data.For example, it may be beneficial to vary the region or depth ofinvestigation in the zone of interest 45 of the formation 30 by changingthe position of the sensing electrodes 15 and utilizing signalprocessing techniques (e.g. switching signal processing modes). Also insome embodiments, the more electrodes 10, 15 included in the system 5,the less axial and rotational movement may be necessary for accurate anduseful data. For example, different sensing electrodes 15 or sets ofsensing electrodes 15 may be positioned at different depths in the wellbore 25 so that each will detect perturbations to a certain depth.During the process of taking multiple perturbation measurements, it maybe desirable to move one or more of the electrodes 10, 15 relative toone or more other electrode 10, 15 or the geological feature to beevaluated to optimize accuracy and usefulness of the results.

As described above, any suitable target objects 35 may be used toproduce the desired data. For example, when the desired data involvesdimensions of fractures in the formation, perturbation data may berecovered based upon the difference in electric conductivity between thefracture (hole) and the formation wall adjacent to the fracture andfluid 28 in the well bore 25. For another example, target objects 35,such as material having a different electrical conductivity, can beadded to deliberately increase the electrical contrast and improve orenhance the perturbation readings.

Depending upon the accuracy or usefulness of the readings or changes inthe downhole conditions, the impedance of the target objects 35 may bechanged. In some instances, it may be desirable to add more or lessconductive material to fracture fluid or proppants inserted into thefracture(s) to create a greater electrical contrast as compared to thefracture fluid itself, the proppant and/or earthen formation 30 aroundthe fracture. Thus, at any stage in the exemplary process, ifperturbation readings are insufficient, target objects 35 may be addedor altered in any desired manner to increase or decrease electricalimpedance and contrast, as necessary. For example, specialized proppantsor particles, such as described above, may be inserted into the wellbore 25 or mixed with fluid or proppants provided in the well bore 25.

Still referring to FIG. 1, the system 5 may be used in mapping orapproximating one or more dimension of one or more fractures (or othergeological feature) in the formation along multiple intervals or anglesin the well bore 25. In some embodiments, the wireline 40 (or othercarrier) may be moved upwardly in the well bore 25 to locate thecorresponding electrodes 10, 15 at a desired second position, such as anext higher fracture interval or area within the same fracture interval.At that location, an electric field may be provided into a new zone ofinterest by the electric field generating electrodes 10 andperturbations from one or more target objects 35 therein may be measuredby the sensing electrodes 25, such as described above. This process maybe repeated at multiple successive locations, as desired, such ascorresponding to hydraulic fracture intervals, pre-determined spacingintervals or based upon any other criteria.

In other embodiments, multiple sets of corresponding electrodes 10, 15may be disposed on the same wireline 40 or other carrier at spacedintervals so that after the wireline 40 or carrier is lowered into thewell, perturbations can be measured at multiple locations withoutcompletely repositioning the wireline 40 or other carrier. In stillfurther embodiments, multiple sets of corresponding electrodes 10, 15may be embedded in, or connected with, a casing (not shown) or othercomponent or fixture in the well bore 25 (such as described above) atdesired intervals to measure perturbations from target objects atdifferent locations.

After data is obtained by the electrodes 15, any methods suitable forprocessing such information and ultimately deducing and/or mapping thedesired dimensions, geometry or spatial relationships from the detectedperturbations as is and become further known may be used. For example, amultitude of different electric potential readings from the sensingelectrodes 15 taken at different times and/or locations may be used tomap out the electric field, formation 30 and/or subject geologicalfeature(s) in a series of iso-potential maps. One or more dataprocessing system (computer or other computing device) may be used. Thedata processing system may be integrated with the electrodes 15, suchas, for example, to dynamically control and vary the location andperformance of the electrodes 10 and/or 15 and to process data. In thepresent embodiment, mathematical modeling techniques, as are and becomefurther known, may be used to formulate and apply appropriate algorithmsvia one or more computing device to determine the relationship betweendetected perturbations and the boundaries or desired dimensions ofassociated fractures. As an example, the above-cited “Solberg”references, all of which are incorporated herein by reference in theentireties, disclose algorithms for determining locations based ondetected electrical perturbations and inversion processing for invertingmeasure perturbations.

EXPERIMENTATION Background

Experiments were conducted to illustrate principals applicable inaccordance with an embodiment of the present disclosure. However, itshould be understood that the present disclosure and appended claims arenot strictly limited to any of the details of the experimentation asdescribed below and shown in the referenced figures.

To experimentally verify the ability to approximate one or moredimension, geometric feature or spatial relationship in a well bore, adevice was built to generate 2V (peak-to-peak) biphasic 1 kHz squarewave. An electric field was generated between two submerged silverelectrodes plated with silver chloride to improve the metal-waterelectrical interface. These electrodes are represented in FIG. 3A as theelectric field generating electrodes 60, which were positionedapproximately 50 mm apart. Two sensing electrodes 64 were positionedabout 50 mm apart, forming an overall diamond pattern with the electricfield generating electrodes 60. The pairs of electrodes 60, 64 made upthe electrode assembly 66 (e.g. FIG. 6). Both the electric fieldgenerating electrodes 60 and the sensing electrodes 64 were constructedof 0.38 mm diameter silver wires that were stabilized by 0.5 mm boratesilicate glass pipettes. However, the electrodes 60, 64 are not limitedto this specific composition, geometry and arrangement, but may beconstructed of other materials and designed and arranged as desired toaccommodate differing conditions, such as in subterranean oilfieldapplications.

FIG. 3A illustrates an exemplary unperturbed planar electric field. Thearrows represent the electric current flowing from the positive electricfield generating electrode 61 to the negative electric field generatingelectrode 62. The contour lines 63 represent lines of equal electricpotential. No target object is present to distort the electric field.The sensing electrodes 64 are positioned where they would give identicalreadings with no target objects present. Both electrodes 64 lie on theiso-potential contour of zero Volts so a zero voltage differentialbetween the sensing electrodes 64 exists. For the purpose ofillustration, FIG. 3B is provided to show the planar electric fieldrepresentation of FIG. 3A perturbed by a circular target object 35, suchas a perforation or hole in the wall of the well bore, centered at x=20and y=−20. The top electrode 64 is reading 78 mV, while the bottomelectrode 64 is reading 122 mV. Thus, in this example, there is adifference in potential between the respective sensing electrodes 64 of−44 mV.

In measuring perturbations caused by one or more target object, thesignals recorded at the sensing electrodes 64 were differentiallyamplified and the resulting signal, along with its negative, was sent toan analog switch. The analog switch passed one of the two input signalsto an output according to a switching signal, which was the originalsquare-wave used to generate the electric field. This served as amatched filter, since only sensory signals having the same frequency asthe field signal have a nonzero time-averaged mean at the output of theanalog switch. The final stage was a low-pass filter that outputs thismean value.

Two computers were used for information processing. One computer ran areal-time operating system (xPC, The Mathworks, Natick Mass. USA) andhandled low-level control of movement and recording and filtering of themeasurements. A second computer received the filtered data from thereal-time computer, generated the next position of the electrodeassembly (readings were taken at multiple locations) and sent suchinformation to the real-time computer. All algorithms were implementedwith commercial software (Simulink, Real Time Workshop, xPC Target, andMATLAB: The Mathworks, Natick Mass. USA).

Experiments were conducted in a 750 mm by 750 mm glass tank 70 (e.g.FIG. 4) filled to a depth of approximately 160 mm. In order to minimizethe effects of the tank walls on the electric field, experiments wereconducted in a central region of 200 mm by 200 mm. Low concentration ofNaCl aqueous solution 74 (e. g. FIG. 4) was used in the experiments,although other types of brines may instead be used. All tests were doneat ambient conditions.

EXPERIMENT #1 Measurement of the Gaps Between Sandstone Rocks

As shown in FIG. 4, two pieces of sandstone rock 76, 78 were placed inthe tank 70 and separated by gap x. The facing sides of the sandstonepieces 76, 78 represented the target objects. This experiment simulatedusing electrolocation to measure the width and height of a crack, suchas a fracture in an earthen formation. The electric field potential wasmeasured (as outlined above) and an isopotential map was produced. Thisprocess was repeated multiple times, each time varying the gap (x value)ranging from 0-16 mm.

FIG. 5 illustrates an example isopotential map that was generated. Theside bar indicates the difference in electric potential in my and the xand y axes correspond to the reading of coordinates. Electricalpotential that was lower than that of the fluid is represented in blue.The areas of lowest potential are represented with concentrated blueareas, or lobe slices 80. Electrical potential that was higher than thatof the fluid is represented in red. The areas of highest potential arerepresented with concentrated red areas, or lobe slices 84.

As shown, the map provides a general outline of the blocks 76, 78 basedupon voltage. A gap “y” is clearly visible between the isopotentialcontours, or lobe slices, 80, 84 in the middle region of the map,representing differences in electrical impedance between the saltwater74 and sandstone pieces 76, 78. The highest potential difference is thusshown generated at the gap y, representing where the blocks 76, 78 wereseparated. The measured gap between the lobe slices 80, 84 isproportional to the actual gap x (FIG. 4) between the sandstone pieces76, 78. Further, the length of the gap y corresponds with the length ofthe gap between the blocks 76, 78. A y-value was obtained in eachsuccessive isopotential map generated after adjusting the gap (x values)in the different runs. Results demonstrated linear relationship betweenx and y, as y∝x. indicating this methodology can be used to determineunderground fracture width and also length.

EXPERIMENT #2 Measurement of Diameters of the Holes in a Sandstone Rock

Referring to FIG. 6, a piece of sandstone rock with a hole 88 was placedin the tank. The electrode assembly 66 was placed in a first position 90above the sandstone rock relative to the hole 88 and the electric fieldpotential was measured (by steps outlined above). The electrode assembly66 was moved to a second position 94, then to a third position 98 etc.to the nth position 100 according to the grid pattern illustrated inFIG. 6 to cover the entire area over the rock, with electric fieldpotential measured at each position.

The information gathered was used to generate the electric fieldisopotential map shown in FIG. 7. The side bar indicates the differencein electric potential in mV and the x and y axes correspond to thereading of coordinates. The distance between the centers (x) of the lobeslices 80, 84, or lobe peak radius (LPR), was measured and averaged.

Multiple shapes of sandstone rock with holes 88 of different diameters(ranging from 4 mm to 16 mm) were tested. Each sandstone rock wasindependently placed in the tank. The electric field potential wasmeasured for each rock and an isopotential map produced. The average LPRwas measured and calculated for each map.

The average LPR and corresponding hole radiuses were plotted in FIG. 8(n=22) and generated the algorithm LPR=0.90*HR+1.71, where HR is thehole radius. The resulting fitted curve and prediction bounds shows aclear linear relationship between the diameter of the hole in each rockand the distance between the mapped lobes, indicating that the distancebetween the peaks x of the lobe slices 80, 84 is proportional to thehole diameter. In an application having a hole in a rock with an unknowndiameter, and the average distance between the lobe centers will lead tothe diameter of the hole. This experiment demonstrated that the diameterof a hole can be determined using electrolocation.

EXPERIMENT #3 Depth Measurements of a Hole or Fracture in SandstoneFormation

In this experiment, two sensing electrodes and the two electric fieldemitter electrodes were embedded 15 mm deep into a sandstone rock placedin the tank. A 7 mm diameter hole was drilled in the center of the rockand 17 mm from each of the four electrodes. The depth of the hole wasincreased gradually and the electric potential was measured at eachincrement of hole depth. An increase in hole depth leads to a change inthe conductivity between electrodes, which creates a voltagedifferential. A relationship between the average voltage differentialbetween electrodes and the depth of the hole was established, as shownin FIG. 9. The value of the hole depth was accurate up to at least threetimes the depth of the electrodes. Such a relationship indicates thatelectrolocation may be useful to measure the depth of a fracture or holein a formation rock.

Preferred embodiments of the present disclosure thus offer advantagesover the prior art and are well adapted to carry out one or more of theobjects of this disclosure. However, the present invention does notrequire each of the components and acts described above and is in no waylimited to the above-described embodiments, methods of operation,variables, values or value ranges. Any one or more of the abovecomponents, features and processes may be employed in any suitableconfiguration without inclusion of other such components, features andprocesses. Moreover, the present invention includes additional features,capabilities, functions, methods, uses and applications that have notbeen specifically addressed herein but are, or will become, apparentfrom the description herein, the appended drawings and claims.

The methods that are provided in or apparent from the description aboveor claimed herein, and any other methods which may fall within the scopeof the appended claims, may be performed in any desired suitable orderand are not necessarily limited to any sequence described herein or asmay be listed in the appended claims. Further, the methods of thepresent invention do not necessarily require use of the particularembodiments shown and described herein, but are equally applicable withany other suitable structure, form and configuration of components.

While exemplary embodiments of the invention have been shown anddescribed, many variations, modifications and/or changes of the system,apparatus and methods of the present invention, such as in thecomponents, details of construction and operation, arrangement of partsand/or methods of use, are possible, contemplated by the patentapplicant(s), within the scope of the appended claims, and may be madeand used by one of ordinary skill in the art without departing from thespirit or teachings of the invention and scope of appended claims. Thus,all matter herein set forth or shown in the accompanying drawings shouldbe interpreted as illustrative, and the scope of the disclosure and theappended claims should not be limited to the embodiments described andshown herein.

1. A method of approximating or determining at least one dimension of atleast one underground geological feature of an earthen formation usingelectrolocation, the at least one geological feature being at leastpartially located within a zone of interest in the earthen formationproximate to a subterranean well bore, the method comprising: while thewell bore and geological feature at least partially contain conductivefluid, generating an electric field in the zone of interest, at leastone target object in the zone of interest having an electrical impedancethat differs from the electrical impedance of the conductive fluid andcreating perturbations in the electric field, providing at least twosensing electrodes in the well bore, the sensing electrodes beingconfigured to detect differences therebetween in electric potentialcaused by the at least one target object, at multiple different timesand multiple different locations in the well bore, the sensingelectrodes detecting differences therebetween in electric potentialcaused by at least one target object and providing data relating theretoto at least one data processing system; and the at least one dataprocessing system approximating or determining at least one dimension ofat least one geological feature in the earthen formation based at leastpartially upon data provided by the sensing electrodes.
 2. The method ofclaim 1 further including moving the sensing electrodes axially androtationally in the well bore; and at each among multiple differentaxial and rotational positions of the sensing electrodes in the wellbore, the sensing electrodes detecting differences therebetween inelectric potential caused by at least one target object.
 3. The methodof claim 1 further including moving the sensing electrodes relative tothe at least one target object; and the sensing electrodes detectingdifferences therebetween in electric potential caused by at least onetarget object at multiple different times and positions of the sensingelectrodes relative to the at least one target object.
 4. The method ofclaim 3 further including providing at least two spaced-apart electricfield generating electrodes in the well bore, the electric fieldgenerating electrodes generating the electric field in the zone ofinterest; and moving the electric field generating electrodes in thewell bore concurrently with moving the sensing electrodes in the wellbore.
 5. The method of claim 4 further including disposing the electricfield generating electrodes and sensing electrodes on a common carrierthat is moveable into, within and out of the well bore.
 6. The method ofclaim 5 further including moving the carrier between multiple positionswithin the well bore to allow the sensing electrodes to provide electricpotential data useful for estimating or determining at least one amongthe height and width of a plurality of geological features in theearthen formation at different intervals or angles along the well bore,wherein at least one among the width and height of each of the pluralityof geological features may be approximated or determined based at leastpartially upon data obtained by the sensing electrodes during a singletrip into the well bore.
 7. The method of claim 1 further includingproviding a plurality of spaced-apart positive electric field generatingelectrodes arranged in a single line along the longitudinal axis of thewell bore; and providing at least one negative electric field generatingelectrode within the well bore, wherein the positive and negativeelectric field generating electrodes generate the electric field in thezone of interest.
 8. The method of claim 7 further includingdirectionally oriented the positive electric field generating electrodesin the direction of at the least one geological feature; and thepositive electric field generating electrodes synchronously injectingcurrent in parallel in the same direction to create the electric fieldover the full length of the at least one geological feature in the zoneof interest, wherein the length of the at least one geological featuremay be approximated or determined based at least partially upon dataobtained by the sensing electrodes.
 9. The method of claim 1 furtherincluding providing a plurality of spaced-apart, positive electric fieldgenerating electrodes arranged in a single line along the longitudinalaxis of the well bore; and providing at least one negative electricfield generating electrode outside the well bore, wherein the positiveand negative electric field generating electrodes generate the electricfield in the zone of interest.
 10. The method of claim I furtherincluding providing a single elongated positive electric fieldgenerating electrode within the well bore and extending along thelongitudinal axis thereof; and providing at least one negative electricfield generating electrode, wherein the positive and negative electricfield generating electrodes generate the electric field in the zone ofinterest.
 11. The method of claim 1 wherein the at least one geologicalfeature includes at least one fracture created in the earthen formationby hydraulic fracturing and extending at least partially into the zoneof interest, further wherein the at least one dimension includes atleast one among the height, width and length of the at least onefracture.
 12. The method of claim 11, further including the sensingelectrodes detecting differences therebetween in electric potentialcaused by at least one target object during first and second detectionphases to provide at least first and second corresponding respectivesets of data, wherein in the first detection phase, the at least onetarget object includes the wall of at least one created fracture and, inthe second detection phase, the at least one target object includesproppant placed into at least one created fracture; and analyzing,processing or comparing the first and second sets of data to assist indetermining at least one among the width and height of the at least onecreated fracture.
 13. The method of claim 12, further including thesensing electrodes measuring electric potential in the electric fieldbefore creation of the at least one fracture and providing a third setof data relating thereto; and comparing the third set of data with atleast one among the first and second sets of data to assist indetermining at least one among the width and height of the at least onecreated fracture.
 14. The method of claim 13, further including thesensing electrodes measuring electric potential in the electric field inone or more area of the zone of interest that does not include at leastone created fracture and providing a fourth set of data relatingthereto; and comparing the fourth set of data with at least one amongthe first, second and third sets of data to assist in determining atleast one dimension of the at least one created fracture.
 15. The methodof claim 11 further including providing functionalized target objectsand wherein the conductive fluid includes fracturing fluid.
 16. Themethod of claim 1 wherein the at least one geological feature includesmultiple fractures created in the earthen formation by hydraulicfracturing and extending at least partially into the zone of interest,further including producing a series of iso-potential maps of at leastpart of the zone of interest to illustrate at least one dimension ofeach of the created fractures.
 17. The method of claim 1 furtherincluding providing a plurality of spaced-apart electric fieldgenerating electrodes in the well bore, the electric field generatingelectrodes generating the electric field; and positioning the sensingelectrodes and electric field generating electrode in fixed positions inthe well bore.
 18. The method of claim 17 further including providingmultiple sets of associated electric field generating and sensingelectrodes disposed at different positions upon or within at least oneamong a casing, cement, insulation or liner; the electric fieldgenerating electrodes configured to create electric fields in multiplelocations in the earthen formation; and the sensing electrodesconfigured to detect differences in electric potential created bymultiple target objects at different locations in the earthen formation.19. The method of claim 1 further including providing a plurality ofspaced-apart electric field generating electrodes in the well bore; theelectric field generating electrodes generating the electric field;affixing at least one among at least one sensing electrode and at leastone electric field generating electrode to a casing in the well bore;and evaluating through-casing resistivity to assist in neutralizing theinfluence of the casing on the electric potential measurements taken bythe sensing electrodes.
 20. The method of claim 1 further including theat least one data processing system using inversion processing to assistin approximating or determining at least one dimension of at least onegeological feature in the earthen formation based at least partiallyupon data provided by the sensing electrodes.
 21. The method of claim 1further including at least one among adding or altering target objectsin the well bore and at least one geological feature to increase thedifference in electrical impedance between the target objects andconductive fluid and improve perturbation readings by the sensingelectrodes.
 22. A system for approximating or determining at least onedimension of at least one geological feature of an earthen formationfrom a subterranean well bore, the at least one geological feature beingat least partially located within a zone of interest in the earthenformation that is proximate to the well bore, the system comprising: aconductive fluid disposed in at least part of the well bore and at leastone geological feature; at least two spaced-apart electric fieldgenerating electrodes configured to create a constant electric fieldextending at least partially into the zone of interest; at least onetarget object disposed within the zone of interest outside the wellbore, having a different electrical contrast as compared to saidconductive fluid and capable of creating perturbations in the electricfield; and at least two spaced-apart sensing electrodes disposed withinthe well bore and configured to detect differences therebetween inelectric potential measured in volts caused by said at least one targetobject and provide data relating thereto for use in approximating ordetermining at least one dimension of at least one geological feature,said sensing electrodes being configured to be movable relative to saidat least one target object in order to detect electric potentialdifferences at different times and locations.
 23. The system of claim 22wherein said at least two electric field generating electrodes includesat least one positive electric field generating electrode disposedwithin the well bore and at least one negative electric field generatingelectrode disposed within or outside the well bore.
 24. The system ofclaim 23 wherein said at least two spaced-apart electric fieldgenerating electrodes includes a plurality of pairs of positive electricfield generating electrodes and negative electric field generatingelectrodes arranged in a single line and moveable into and within thewell bore, further including a distinct said sensing electrodecorresponding with each said pair of positive and negative electricfield generating electrodes, wherein said electric field generatingelectrodes and said sensing electrodes are configured to be concurrentlymoveable within the well bore and relative to said at least one targetobject.
 25. The system of claim 24 wherein the at least one geologicalfeature is a hydraulic fracture and said at least one target objectincludes the wall of the hydraulic fracture during a first detectionphase of said sensing electrodes and proppant placed into the hydraulicfracture during a second detection phase of said sensing electrodes. 26.The system of claim 24 wherein said electric field generating electrodesand said sensing electrodes are disposed upon a common carrier.
 27. Thesystem of claim 23 further including at least fifteen said electricfield generating electrodes and at least ten said sensing electrodes.28. The system of claim 23 further wherein said positive electric fieldgenerating electrodes are configured to synchronously inject current inparallel in the same direction to create the electric field over thefull length of the at least one geological feature in the zone ofinterest, wherein the length of the at least one geological feature maybe approximated or determined based at least partially upon dataobtained by said sensing electrodes.
 29. The system of claim 22 whereinsaid at least two spaced-apart electric field generating electrodesincludes at least one elongated positive electric field generatingelectrode disposed within the well bore and at least one negativeelectric field generating electrode not disposed within the well bore.